In mining, or like operations, it has been found that mine roof bolts inserted in roof openings, and fully grouted in resin or cement, very often provide better roof control than tensioned bolts. For many years, wooden dowels have been used in situations where a wet condition of the roof exists, with the result that the water causes the wood to expand and grip the walls of the hole throughout its length, thus providing satisfactory roof strengthening.
Various means of producing the desired bonding effect between structural members and the roof have been tried such as a split set which is forced into the hole by hammer blows, and a worley bolt which embodies a solid steel rod which is driven into the roof. These devices do not, however, produce the bond strength necessary under certain mining conditions, or are unsatisfactory for other reasons.
More recently, a circumferentially compressible member such as disclosed in U.S. Pat. Nos. 3,922,867 and 4,012,913 have been employed for the present purpose, but it has been found that better bonding results are obtainable with members which are in tension after installation rather than compression.
An example of a member held in tension is disclosed in U.S. Pat. No. 4,147,444 which relates to an expansion dowel having a tubular member which is longitudinally split, and forced apart by a single spreader into engagement with the roof in which it is inserted. A very limited and non-uniform expansion of the number is effected with this arrangement.